Method and arrangements in a telecommunication system

ABSTRACT

Methods and arrangements in a communication system are described for Multi User Multiple-Input-Multiple-Output (MU-MIMO) signaling via Multiple-Input-Multiple-Output (MIMO) antennas between a base station and one of a plurality of mobile terminals supporting both Single User and Multi User Multiple-Input-Multiple-Output (SU-MIMO and MU-MIMO) signaling modes. Switching between the modes is supported in the system and the modes have partly shared signaling. SU-MIMO mode signaling which is redundant for MU-MIMO mode signaling is identified. Data bits of the identified redundant signaling is re-interpreted or re-defined to comprise signaling information associated with MU-MIMO mode. The signaling information associated with MU-MIMO mode is signaled to the mobile terminal using the reinterpreted redundant bits.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation of U.S. patent application Ser. No.14/636,124, filed Mar. 2, 2015, which is a continuation of U.S. patentapplication Ser. No. 12/666,115, filed Jul. 2, 2010 (now U.S. Pat. No.8,989,290), which is a U.S. national stage entry under 35 U.S.C. §371 ofInternational Application No. PCT/SE2008/050383, filed Apr. 3, 2008 (andpublished on Dec. 31, 2008 in English by the International Bureau asInternational Publication No. WO 2009/002251), which claims priority toSwedish Application No. 0701582-9, filed Jun. 27, 2007. The entirecontents of each of the above-referenced patent applications are herebyincorporated by reference.

BACKGROUND

The present invention relates to a method and arrangements in atelecommunication system. In particular it relates to a method andarrangements for Multi User Multiple-Input-Multiple-Output signaling inthe telecommunication system.

In the cellular telecommunication system which typically comprises, asillustrated in FIG. 1, a core network 1, a radio access network 2, UserEquipments (UEs) 4 and base stations 3 multiple transmit antennas can beused for achieving high data rates in various ways. Amultiple-input-multiple-output (MIMO) channel is formed if the receiveralso has multiple antennas. One application in such a setup is to strivefor high peak rates to a single user. By transmitting on several layers,i.e., wherein the information is transmitted on several bit streams theinformation is spread in the spatial domain, and substantial improvementin data rate can be achieved under favorable channel conditions. This iscalled single user MIMO (SU-MIMO) since the data on several layers isintended for a single receiver/user/UE/terminal. FIG. 2 shows an exampleof a base station 20, with multiple transmit antennas 23, that istransmitting in SU-MIMO mode to a single UE 21. As shown in FIG. 2several layers 22 are transmitted to a single UE 21. In FIG. 2 the UE isalso transmitting to the base station 20 using several layers. Thetelecommunication system may be an LTE-system which is an evolution ofthe UMTS.

The number of simultaneously transmitted layers depends highly on theproperties of the MIMO channel. Because of, for example fading, usuallythe MIMO channel does not support more than one layer transmission to asingle UE. This limits the data rate and means that spatial multiplexinggain is not possible. To still reach high system capacity it might bebeneficial to transmit only a limited number of layers to a single userand instead schedule several users on the same physical resource (e.g.time-frequency-code tile) and use the spatial domain (layers) toseparate the users. In essence, layers belonging to different users aretransmitted on the same physical resource. Even if the channel to aparticular user is such that it does not support multiple layers, whichmeans that it is not possible to transmit multiple layers to thatparticular user, spatial multiplexing gain on a system level can beachieved as long as the user can efficiently suppress the layerstransmitted to the other users. This technique is sometimes referred toas multi-user MIMO (MU-MIMO) and is especially attractive in high loadscenarios with many active users as described in 3GPP R1-063130, “Systemlevel comparison between MU- and SU-MIMO for downlink precoding systemswith four transmit antennas”, Ericsson. TSG-RAN WG I #47, Nov 2006. FIG.3 shows an example of a base station 20, with multiple transmit antennas23, that is transmitting in MU-MIMO mode to multiple UEs 31, 32 and 33.As shown in FIG. 3, different layer 34, 35 and 36 is transmitted to eachUE 31, 32 and 33. As illustrated in FIG. 3, each UE is also transmittingto the base station 20 using different layers.

In the Long Term Evolution (LTE) standardization process, there isagreement on the support of SU-MIMO and MU-MIMO in the downlink and thatthere will be the possibility to semi-statically switch between thesetwo modes. Each UE (user/receiver) can in the MU-MIMO mode receive zeroor one layer. There are basically three proposals for MU-MIMO supportunder discussion:

-   -   1. Classical space-division multiple access (SDMA) for        correlated antenna array setups described in 3GPP R1-072464,        “MU-MIMO for E-UTRA DL”, Ericsson, TSG-RAN WG1 #49, May 2007.    -   2. Zero-forcing beamforming described in 3GPP R1-071510,        “Details of Zero-Forcing MU-MIMO for DL E-UTRA”, Freescale        Semiconductor Inc., TSG RAN WG 1 #48bis, March 2007.    -   3. Per user unitary rate control (PU2RC) described in 3GPP        R1-060335, “Downlink MIMO for EUTRA”, Samsung, TSG RAN WG1 #44,        February 2006.

It has not yet been decided which scheme to support and many remainingdetails of the standardization of MU-MIMO are hence lacking. One problemthat occurs regardless of MU-MIMO mode is how the UE is to know thepower offset between a power reference, for instance reference symbols(RS), and data symbols transmitted to the UE. The base station has acertain transmit power of which a certain amount is used to transmitdata symbols to a particular UE. The power offset indicates how muchpower is used to transmit the data symbols in relation to the powerreference. This power offset is needed in order to support efficientdemodulation in the UE when higher order modulation alphabets like 16QAM or other higher order modulation schemes are used. The power offsetcan vary dynamically because of different power settings at the basestation (also referred to as Node B or eNode B). In case of MU-MIMO, thepower offset can however also fluctuate because of a varying number ofmultiplexed UEs on the same physical resource. The availabletransmission power may for example be equally divided among UEsscheduled on different layers in the MU-MIMO mode, meaning less powerper UE when several UEs are multiplexed.

SUMMARY

It is therefore an object of the present invention to provide animproved solution for MU-MIMO signaling in a telecommunication systemfor obviating at least some of the above mentioned problems. Theinventive solution to this end intends to enable reuse of SU-MIMOfunctionality as much as possible in order to support MU-MIMO operationand to reinterpret any signaling in SU-MIMO mode that is not needed forMU-MIMO.

More specifically a first aspect of the present invention relates to amethod for Multi User Multiple-Input-Multiple-Output (MU-MIMO) signalingfrom Multiple-Input-Multiple-Output MIMO antennas of a base station toone of a plurality of mobile terminals of a communication system, wherethe mobile terminal is configured for both Single User and Multi UserMultiple-Input-Multiple-Output (SU-MIMO and MU-MIMO) signaling modes.The mobile terminal is also configured to switch between the modes,wherein the modes have partly shared signaling. A first step of themethod comprises identifying SU-MIMO signaling, which is redundant forthe MU-MIMO mode. A second step in the method comprises reinterpretingbits of the redundant signaling to comprise signaling information,associated with MU-MIMO mode. A third step in the method is to informthe UE via signaling about the reinterpretation of the redundant bits.The method further comprises a fourth step of signaling informationassociated with MU-MIMO mode by using the reinterpreted redundant bits.

A second aspect of the present invention relates to a radio base stationin a communication system comprising Multiple-Input-Multiple-Output(MIMO) antennas for signaling to one of a plurality of mobile terminalsand configured to both Single User and Multi UserMultiple-Input-Multiple-Output (SU-MIMO and MU-MIMO) signaling modes,where the base station comprises means for switching between the modes.The modes having partly shared signaling and the base station furthercomprises, a signaling mode comparator means for identifying, SU-MIMOmode signaling which is redundant for MU-MIMO mode. A reinterpretingmeans is also present for reinterpreting bits of the redundant signalingto comprise signaling information, associated with MU-MIMO mode. Furthermeans in the base station is a transmitter means for informing viasignaling to the UE about the reinterpretation of the redundant bits andsignaling information associated with MU-MIMO mode by using thereinterpreted redundant bits.

A third aspect of the present invention relates to a mobile terminalcomprising Multiple-Input-Multiple-Output (MIMO) antennas configured forboth Single User and Multi User Multiple-Input-Multiple-Output (SU-MIMOand MU-MIMO) signaling modes wherein the mobile terminal comprises meansfor switching between the modes, the modes having partly sharedsignaling. The mobile terminal further comprises a receiver means forreceiving from the base station information about a reinterpretation ofredundant SU-MIMO bits. By using the receiver means, the mobile terminalalso receives signaling information associated with MU-MIMO mode byreceiving the reinterpreted redundant bits. There is also means forinterpreting the reinterpreted redundant bits using the informationabout the reinterpretation of redundant SU-MIMO bits.

The invention has the advantage of enabling simpler implementation andfewer options in the communication system. The latter is particularlyimportant for facilitating testing that equipment complies withstandard. By possibly using codebook subset restriction in combinationwith intentionally scheduling several terminals or UEs on the samephysical resources (resource elements), MU-MIMO can be efficientlysupported, at least for SDMA (Space Division Multiple Access) andcorrelated antenna arrays, when the terminals or UEs are informed of thepower offset between the power reference and own data symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cellular telecommunication system wherein thepresent invention may be implemented.

FIG. 2 illustrates a base station operating in SU-MIMO mode.

FIG. 3 illustrates a base station operating in MU-MIMO mode.

FIG. 4 illustrates bits used for signaling in SU-MIMO mode and MU-MIMOmode.

FIG. 5 illustrates a method according to embodiments of the presentinvention.

FIG. 6 illustrates a base station according to embodiments of thepresent invention.

FIG. 7 illustrates a mobile terminal according to embodiments of thepresent invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following detailed description ofpreferred embodiments.

The present invention relates to a cellular telecommunication system asillustrated in FIG. 1. The present invention sets forth a method andarrangements for Multi User Multiple-Input-Multiple-Output signaling ina telecommunication system. The basic idea of the present invention isto utilize the fact that MU-MIMO and SU-MIMO share many similarities.However they are currently classified as two quite different modes, butthere is a lot to gain from exploiting the commonality between the twomodes for reducing the number of signaling options. This would not onlymean simpler implementation of transmitters and receivers but it willalso facilitate testing of equipment to ensure that it fulfills thestandard.

When transmitting in SU-MIMO mode or MU-MIMO mode a precoding matrixneeds to be selected by the base station for the transmission. Accordingto 3GPP standard the precoding matrix shall be selected from precodingmatrices in a 3GPP Standard code book. Each UE recommends, among theprecoding matrixes in the codebook, a subset of precoding matrix fromthe codebook to be used by the Node B in possible subsequent downlinktransmissions. Based on e.g. the antenna configuration of the basestation the base station can further confine the precoding matricesselection in the UE to a subset of the precoding matrices in thecodebook. This technique is called codebook subset restriction. Usingcode book subset restriction means that the base station has to signalfewer alternative precoding matrices to the UE from which the UE canrecommend one to the base station.

By utilizing the agreed working assumption of codebook subsetrestriction, almost everything from SU-MIMO mode can be reused forMU-MIMO mode as long as it is not explicitly forbidden to schedulemultiple terminals or UEs on the same physical resource. One thing thatneeds to be added to the MU-MIMO mode is informing the UE at a fast rateabout the previously mentioned power offset between its data symbols andthe power reference. This minor modification seems at least sufficientfor SDMA for correlated antenna arrays and a number of related setups.An alternative to signaling the power offset to the UEs is to signal thetotal number of multiplexed UEs.

The present invention adds support for MU-MIMO mode signaling such aspower offset signaling by utilizing SU-MIMO signaling (i.e. bits) thatis not needed for MU-MIMO operation and letting them be re-interpretedas MU-MIMO mode signaling information such as the power offsetsignaling. Note that the signaled power offset can be relative to othervarying power offset levels such as data to pilot power ratios and/ormeasurement power offsets. The transmission rank, i.e. the number oftransmitted layers is one example of such unused SU-MIMO signaling sincethe MU-MIMO mode only needs to support a limited transmission rank perUE. Bits used for precoding matrices not needed because of e.g.application of codebook subset restriction can also bereused/reinterpreted.

Taking four transmitting antennas at the NodeB as an example, signalingof the transmission rank in SU-MIMO mode requires essentially two bits.The transmission rank is signaled in the downlink whenever the UE isscheduled, i.e. possibly for each sub frame. These bits can be reusedwhen transmitting in MU-MIMO mode to e.g. indicate a four-level poweroffset instead of the transmission rank. The same or a differentgranularity between the power offset steps may be used. In the simplestcase, the power offsets correspond to the factors ¼, ⅓, ½ and 1corresponding to four, three, two and one multiplexed UEs respectively.This would for example be applicable to a scenario using Space DivisionMultiple Access (SDMA) with eight beamformers and a uniform linear array(ULA) having 0.5 wavelength element spacing, since this antennaconfiguration makes it possible to multiplex up to four UEs on thephysical resources.

Similarly, for the case with two transmitting antennas up to two UEs canbe multiplexed in MU-MIMO mode and there is hence a need to signal poweroffset factors ½ and 1. In the case of correlated SDMA, four differentbeamforming/precoding matrices are typically needed, since the otherprecoding matrices in the 2 Tx LTE codebook are not relevant for thisantenna configuration. The 2 Tx codebook in LTE has totally nineelements, which means that five of the elements never need to besignaled to the UE.

As illustrated in FIG. 4 four bits 40 are reserved in SU-MIMO mode forsignaling from the base station to the UE which precoding matrix thatthe UE should choose from. In the case of MU-MIMO mode only two bits 41are needed for signaling to the UE which precoding matrix that the UEshould choose from. In MU-MIMO mode the two bits 42 are thereforeredundant and may therefore be used to signal the power offset to theUE.

A 4 Tx NodeB antenna setup divided into two pairs of cross-polarizedantennas is another important scenario that may be beneficial forMU-MIMO operation. If the antenna pairs are spaced 0.5 wavelengthsapart, it is possible to perform correlated beamforming to separate theusers and to utilize the two orthogonal polarizations for supporting upto two layer transmission for each UE. The 4 Tx SU-MIMO codebook hasbeen concluded in the LTE standardization process, to contain 16elements per transmission rank (number of transmitted layers). A minimalcodebook for MU-MIMO in the considered setup would need four precoderelements when implementing 2 Tx beamforming on each polarization forsingle layer transmission and two precoder elements for dual layertransmission. For single rank transmission, a two bit power scalingoffset is needed while for dual layer transmission, only a single bitpower scaling is needed. This leads to a total of 4*4+2*2=20combinations, which fit well into the 64 different combinations ofprecoder elements and rank in the current SU-MIMO codebook. The poweroffset may be combined with other, possibly dynamically varying, poweroffsets to form the overall power ratio.

Referring to FIG. 5, one aspect of the present invention relates to amethod in a radio base station in a communication system for Multi UserMultiple-Input-Multiple-Output (MU-MIMO) signaling viaMultiple-Input-Multiple-Output (MIMO) antennas between the base stationand one of a plurality of multiplexed mobile terminals supporting bothSingle User and Multi User Multiple-Input-Multiple-Output (SU-MIMO andMU-MIMO) signaling modes. The MU-MIMO modes can for instance be SDMA,zero-forcing beamforming or per user unitary rate control (PU2RC). Thecommunication system supports switching between the modes which alsohave partly shared signaling. The method, which is shown in theflowchart in FIG. 5, comprises the steps of: (501) identify SU-MIMO modesignaling which is redundant, i.e. not needed, for a MU-MIMO modesignaling; (502) re-interpret or re-define bits i.e. data bits, of theidentified redundant signaling to comprise signaling information,associated with MU-MIMO mode; (503) inform the UE about thereinterpretation of the redundant bits; and (504) signal informationassociated with MU-MIMO mode by using said reinterpreted redundant bits.

In one embodiment of the method according to the present invention, theredundant SU-MIMO mode signaling relates to transmission rank signalingand precoder element signaling. According to one embodiment of themethod according to the present invention the identified redundantSU-MIMO mode signaling information is reinterpreted to comprise a poweroffset between a power reference and data symbols of said one terminal.The power reference can for instance be reference symbols. Theidentified redundant signaling information can also in anotherembodiment be reinterpreted to comprise the total number of themultiplexed terminals. The reinterpreted MU-MIMO mode bits may besignaled whenever the terminal is scheduled.

Another aspect of the present invention relates to a radio base station60 in a communication system. The base station 60, which is shown inFIG. 6. has Multiple-Input-Multiple-Output (MIMO) antennas 65 forenabling signaling to at least one of a plurality of multiplexed mobileterminals (not shown) and supports both Single User and Multi UserMultiple-Input-Multiple-Output (SU-MIMO and MU-MIMO) signaling modes.The modes may be SDMA for correlated antenna arrays setups, zero-forcingbeamforming or per user unitary rate control (PU2RC) or other MU-MIMOmodes. The base station further comprises means 61 for switching betweensaid modes and signaling mode comparator means 62 for identifyingredundant SU-MIMO mode signaling that can be reinterpreted to be usedwith MU-MIMO mode. The redundant SU-MIMO mode signaling can for instancerelate to transmission rank signaling and/or precoder element signaling.The radio base station 60 also has reinterpreting means 63 forredefining bits of the redundant signaling to comprise signalinginformation, associated with the MU-MIMO mode. The MU-MIMO signalinginformation may comprise a power offset between a power reference (e.g.reference symbols (RS)) and the power used to transmit data symbols.Alternatively the MU-MIMO signaling information comprises the totalnumber of multiplexed terminals. The base station also comprisestransmitter means 64 for informing the UE about the reinterpretation ofthe redundant bits and signaling information associated with MU-MIMOmode by using the reinterpreted redundant bits. The MU-MIMO modesignaling information could for instance be the power offset or thenumber of terminals that are multiplexed on the physical resource.

Yet another aspect of the present invention relates to mobile terminal70. The mobile terminal 70, which is shown in FIG. 7, hasMultiple-Input-Multiple-Output (MIMO) antennas 74 and supports bothSingle User and Multi User Multiple-Input-Multiple-Output (SU-MIMO andMU-MIMO) signaling modes. The modes may be SDMA for correlated antennaarrays setups, zero-forcing beamforming or per user unitary rate control(PU2RC) or other MU-MIMO modes. The mobile terminal 70 further comprisesmeans 71 for switching between the modes. Redundant SU-MIMO modesignaling can for instance relate to transmission rank signaling and/orprecoder element signaling. Receiver means 72 is also provided in themobile terminal 70 for receiving from a base station (not shown)information about a reinterpretation of redundant SU-MIMO bits. Thereceiver means 72 is further configured to receive signaling informationassociated with MU-MIMO mode by receiving the reinterpreted redundantbits. The MU-MIMO signaling information may comprise a power offsetbetween a power reference (e.g. reference symbols (RS)) and the powerused to transmit data symbols. Alternatively, the MU-MIMO signalinginformation may comprise the total number of multiplexed terminals. Themobile terminal also comprises means 75 for interpreting the receivedreinterpreted redundant bits using the information about thereinterpretation of redundant SU-MIMO bits.

The invention herein disclosed provides significant advantages byproposing a way for supporting MU-MIMO by reusing as much as possibly ofthe signaling functionality from SU-MIMO mode. By using codebook subsetrestriction in combination with intentionally scheduling several UEs onthe same physical resources (resource elements). MU-MIMO can beefficiently supported, at least for SDMA and correlated antenna arrays,when the UEs are informed at a high rate about the power offset betweenthe power reference and own data symbols.

The invention is however not limited to SDMA and correlated antennaarrays as it may be applicable regardless of MU-MJMO mode.

The high reuse of signaling enables simpler implementation and providesfewer options in the system. The latter is particularly important forfacilitating testing that equipment complies with standard.

Note that although terminology from 3GPP LTE has been used in thisdisclosure to exemplify the invention, this should not be seen aslimiting the scope of the invention to only the aforementioned system.Other wireless systems may also benefit from exploiting the ideascovered within this disclosure. Furthermore, the invention is notnecessarily restricted to downlink as it may be applicable for uplinktransmission techniques as well.

Means mentioned in the present description can be software means,hardware means or a combination of both.

While the present invention has been described with respect toparticular embodiments (including certain device arrangements andcertain orders of steps within various methods), those skilled in theart will recognize that the present invention is not limited to thespecific embodiments described and illustrated herein. Therefore, it isto be understood that this disclosure is only illustrative. Accordingly,it is intended that the invention be limited only by the scope of theclaims appended hereto.

What is claimed is:
 1. A method of signaling a mobile terminal from abase station, the mobile terminal being operable in a Single UserMultiple-Input-Multiple-Output (SU-MIMO) mode and a Multi UserMultiple-Input-Multiple-Output (MU-MIMO) mode, the method comprising:identifying at least one SU-MIMO signaling bit associated with SU-MIMOsignaling that is not needed in the MU-MIMO mode; redefining the atleast one SU-MIMO signaling bit to be associated with MU-MIMO signaling;informing the mobile terminal about the redefinition of the at least oneSU-MIMO signaling bit; and signaling the mobile terminal in the MU-MIMOmode using at least one MU-MIMO signaling bit, the at least one MU-MIMOsignaling bit comprising the redefined at least one SU-MIMO signalingbit.
 2. The method of claim 1, wherein the SU-MIMO and MU-MIMO modeshave partly shared signaling.
 3. The method of claim 1, wherein the atleast one SU-MIMO signaling bit is associated with transmission ranksignaling and/or precoder element signaling in the SU-MIMO mode.
 4. Themethod of claim 1, wherein the redefined at least one SU-MIMO signalingbit is associated with a downlink power offset in the MU-MIMO mode. 5.The method of claim 4, wherein the power offset is between a powerreference and data symbols.
 6. The method of claim 5, wherein said powerreference is reference symbols (RS).
 7. The method of claim 1,comprising signaling the mobile terminal in the MU-MIMO mode using theat least one MU-MIMO signaling bit whenever the mobile terminal isscheduled.
 8. A base station comprising: Multiple-Input-Multiple-Output(MIMO) antennas for signaling a mobile terminal; a processor configuredto: identify at least one SU-MIMO signaling bit associated with SU-MIMOsignaling that is not needed in the MU-MIMO mode; and redefine the atleast one SU-MIMO signaling bit to be associated with MU-MIMO signaling;a transmitter configured to: inform the mobile terminal about theredefinition of the at least one SU-MIMO signaling bit; and signal themobile terminal in the MU-MIMO mode using at least one MU-MIMO signalingbit, the at least one MU-MIMO signaling bit comprising the redefined atleast one SU-MIMO signaling bit.
 9. The base station of claim 8, whereinthe SU-MIMO and MU-MIMO modes have partly shared signaling.
 10. The basestation of claim 8, wherein the at least one SU-MIMO signaling bit isassociated with transmission rank signaling and/or precoder elementsignaling in the SU-MIMO mode.
 11. The base station of claim 8, whereinthe redefined at least one SU-MIMO signaling bit is associated with adownlink power offset in the MU-MIMO mode.
 12. The base station of claim11, wherein the power offset is between a power reference and datasymbols.
 13. The base station of claim 11, wherein said power referenceis reference symbols (RS).
 14. The base station of claim 8, wherein thetransmitter is configured to signal the mobile terminal in the MU-MIMOmode using the at least one MU-MIMO signaling bit whenever the mobileterminal is scheduled.
 15. A method for a mobile terminal operable in aSingle User Multiple-Input-Multiple-Output (SU-MIMO) mode and a MultiUser Multiple-Input-Multiple-Output (MU-MIMO) mode, the methodcomprising the steps of: receiving information from a base station, theinformation redefining at least one SU-MIMO signaling bit to beassociated with MU-MIMO signaling, the at least one SU-MIMO signalingbit having been identified by the base station as being associated withSU-MIMO signaling that is not needed in the MU-MIMO mode; receiving atleast one MU-MIMO signaling bit in the MU-MIMO mode, the at least oneMU-MIMO signaling bit comprising the redefined at least one SU-MIMOsignaling bit; and interpreting said received redefined at least oneSU-MIMO signaling bit in the MU-MIMO mode based on the receivedinformation.
 16. The method of claim 15, wherein the SU-MIMO and MU-MIMOmodes have partly shared signaling.
 17. The method of claim 15, whereinthe at least one SU-MIMO signaling bit is associated with transmissionrank signaling and/or precoder element signaling in the SU-MIMO mode.18. The method of claim 15, wherein the redefined at least one SU-MIMOsignaling bit is associated with a downlink power offset in the MU-MIMOmode.
 19. The method of claim 18, wherein the power offset is between apower reference and data symbols.
 20. The method of claim 19, whereinsaid power reference is reference symbols (RS).
 21. The method of claim15, comprising receiving the at least one MU-MIMO signaling bit in theMU-MIMO mode whenever the mobile terminal is scheduled.
 22. A mobileterminal configured to operate in a Single UserMultiple-Input-Multiple-Output (SU-MIMO) mode and a Multi UserMultiple-Input-Multiple-Output (MU-MIMO) mode, the mobile terminalcomprising: a receiver configured to receive from a base station:information redefining at least one SU-MIMO signaling bit to beassociated with MU-MIMO signaling, the at least one SU-MIMO signalingbit having been identified by the base station as being associated withSU-MIMO signaling that is not needed in the MU-MIMO mode; and at leastone MU-MIMO signaling bit in the MU-MIMO mode, the at least one MU-MIMOsignaling bit comprising the redefined at least one SU-MIMO signalingbit; and a processor configured to interpret said received redefined atleast one SU-MIMO signaling bit in the MU-MIMO mode based on thereceived information.
 23. The mobile terminal of claim 22, wherein theSU-MIMO and MU-MIMO modes have partly shared signaling.
 24. The mobileterminal of claim 22, wherein the at least one SU-MIMO signaling bit isassociated with transmission rank signaling and/or precoder elementsignaling in the SU-MIMO mode.
 25. The mobile terminal of claim 22,wherein the redefined at least one SU-MIMO signaling bit is associatedwith a downlink power offset in the MU-MIMO mode.
 26. The mobileterminal of claim 25, wherein the power offset is between a powerreference and data symbols.
 27. The mobile terminal of claim 26, whereinsaid power reference is reference symbols (RS).
 28. The mobile terminalof claim 22, wherein the receiver is configured to receive the at leastone MU-MIMO signaling bit in the MU-MIMO mode whenever the mobileterminal is scheduled.